The invention arises from the measurement of forces, friction forces in particular, acting on a moving object in a guide and inaccessible to an operator. The main application of this method is the measurement of forces acting on a mobile assembly formed of the control rod and cluster of control rodlets commonly called: xe2x80x9ccontrol clusterxe2x80x9d, inserted in the fuel assembly of a pressurized water nuclear reactor.
The neutron activity in the core of a pressurized water reactor is regulated by the controlled use of control clusters inserted downwards into fuel assemblies to shut down nuclear activity. The safety of pressurized water reactors therefore partly depends upon the proper functioning of the control clusters. For example, an emergency stoppage of the nuclear reactor requires the dropping under gravity of the control clusters within a given time period, in the order of two seconds but depending on the type of reactor. Each cluster descends into the core of the reactor under gravity, the travel distance in the order of four metres being finally halted by a buffer.
During required programmed maintenance operations, to change fuel rod assemblies or control clusters, tests are needed to conduct measurements in order to control the functioning of these control clusters. A certain number of operating circumstances, such as deformation of fuel assemblies, wear of control cluster guides, the presence of foreign bodies, deformation or swelling of the control cluster or control rods, excessive flow rate in the fuel assemblies, transverse flow of water in the plenum, may deteriorate the dropping under gravity of the control clusters and therefore accelerate or delay their downward movement or even cause its obstruction. It follows that it is therefore necessary and essential to have permanent knowledge of induced forces whether parasitic or not, under these special or deteriorating conditions, and their position along the travel pathway.
However this entire mechanism of control clusters, their guide and their control rod, is located within the very containment of the core of the reactor and is therefore inaccessible to human operators. Similarly, materials including this assembly of reactor equipment, are installed within this containment. It is therefore impossible to control their functioning or to ascertain degradation through human action or by retrieval of the equipment to be inspected.
One measurement device that is very frequently used to control the movement of control clusters is a rod position indicator (RPI), able to measure either movement or speed of movement. It is to measure speed of movement that the RPI is used in the event described above.
It has been customary, up until the date of filing of the present patent application, to measure and record the dropping speed of the cluster and its end-of-travel slowing. Should a substantial increase in the slowdown time be recorded, in accordance with preset criteria, a decision is taken to change the faulty equipment. Laboratory tests, non contaminated, may help to improve these measurements and controls but cannot reproduce the exact conditions prevailing within the containment of the reactor core.
Moreover, measurement of the change in dropping speed (called xe2x80x9cfall kinematicsxe2x80x9d) remains approximate using this rod position indicator (RPI).
The purpose of the invention is to overcome these disadvantages by determining a method for evaluating or determining the friction forces acting on the lowering of a control cluster, this method being reliable for managing the operating efficiency of the control clusters and optionally organizing their change or replacement or the change and replacement of fuel assemblies. The achievement of this purpose will therefore provide improved management of the core of a pressurized water reactor.
For this purpose, the main subject of the invention is a method for determining friction forces occurring on a moving object in a guide, by means of a speed sensor, and comprising the following steps:
1) measuring and recording changes in the velocity of the object initially before the occurrence of friction:
V1=f1(t)
xe2x80x83wherein V1 represents the velocity of the object before friction occurs, and f1 is a function of time (t).
2) calculating distance of travel d1 with integrated change in velocity of the object before the onset of friction, to obtain the change in velocity V1=g1(d) in relation to distance of travel:                     d        1            ⁢              (        t        )              =                  ∫                  u          =          0                t            ⁢                        V          ⁢                      (            u            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          u                      ;
xe2x80x83wherein g1 is a second function of distance (d), and (u) is a time value.
3) measuring and recording the change in velocity of the object after the onset of friction:
V2=f2(t);
4) calculating distance of travel with integrated change in velocity of the object, after the onset of friction, to obtain the change in this velocity V2=g2(d) in relation to distance of travel:                     d        2            ⁡              (        t        )              =                  ∫                  u          =          0                t            ⁢                                    V            2                    ⁡                      (            u            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          u                      ;
5) calculating the difference between the two velocitaies in relation to travel before and after onset of additional friction V3=(g1xe2x88x92g2) (d)=g3(d);
6) calculating the change in velocity of the object in relation to travel, before the onset of friction, using a predetermined calculation programme:
V4=g4(d)
7) subtracting from this change V4 the difference V3 in measured speed changes, V5=V4xe2x88x92V3=g5(d); and
8) deducing, by differentiation between V4 and V5 and multiplication by the weight M, the additional friction forces acting on the movement of the object:
Fadditional=M(xcex35xe2x88x92xcex34)=f(d).
xe2x80x83in which             γ      i        =                  ⅆ                  V          i                            ⅆ        t              ,
xe2x80x83xcex3 is the acceleration of the object, and the subscripts 2, 3, 4, 5 associated with each symbol represent four successive values of the calculating process.
The chief application concerns measurement of the dropping of a rod control cluster assembly into a pressurized water nuclear reactor, the moving object being a control rod and a control cluster, the speed sensor being a rod position indicator and the guide being the lowering channel.